The present invention relates to an active anti-vibration apparatus and, more particularly, to an active anti-vibration apparatus, having a vibration detector and an actuator, such as an air spring and a linear motor, which is used as a support mechanism in precision equipments, such as a semiconductor exposure apparatus.
Along with advances of precision equipments, such as an electron microscope and a semiconductor exposure apparatus, higher performance is required of a precision anti-vibration apparatus for mounting the equipment. Especially, in a semiconductor exposure apparatus, an anti-vibration table for removing vibrations, as much as possible, transmitted from outside the apparatus, e.g., vibration of the setting floor, is necessary for an appropriate and quick exposure operation. This is because vibration, which is harmful to an exposure operation, should be prevented from occurring in an exposure stage.
The semiconductor exposure apparatus has as its feature an intermittent operation called "step and repeat". Repeated step movement of an X-Y stage in this operation excites vibrations of an anti-vibration table. More specifically, reaction force of the X-Y stage movement and displacement of the X-Y stage excite vibrations. Accordingly, an anti-vibration table is required to have anti-vibration performance for removing vibrations transmitted from outside the apparatus, e.g., vibration of the setting floor, and vibration control performance for controlling vibrations due to operation of an equipment mounted on the anti-vibration table.
As for a method replacing the step and repeat operation, there is a scan exposure method often adopted by a semiconductor exposure apparatus. In the semiconductor exposure apparatus, it is necessary to remove vibrations transmitted from outside the apparatus, e.g., vibration of the setting floor, as much as possible, as well as instantaneously control vibrations of the anti-vibration table caused by scanning operation of an exposure stage. Particularly, since exposure is performed while the exposure stage is performing scanning in a scan exposure apparatus, high performance for eliminating vibrations transmitted from outside the apparatus and high performance for controlling vibrations due to operation of an equipment mounted on the anti-vibration table are demanded. Accordingly, an anti-vibration apparatus of high performance is required.
To meet this requirement, an active anti-vibration apparatus has been put in practice where vibrations of the anti-vibration table are detected by vibration sensors, and output signals from the vibration sensors are fed back to actuators used for controlling the anti-vibration table, thereby performing vibration control of the anti-vibration table. The active anti-vibration apparatus allows realization of anti-vibration performance and vibration control performance in a good balance, which is difficult to realize by a conventional passive anti-vibration apparatus constituted by a spring and a damper.
In a conventional active anti-vibration apparatus, in general, a control loop is configured for each pair of a vibration sensor, provided on an anti-vibration table, and an actuator provided at the nearest position of the vibration sensor, and controls vibrations, position and posture. In other words, a plurality of independent control loops are provided for different supporting positions of the anti-vibration table. FIG. 8 shows a configuration of a conventional active anti-vibration apparatus. An anti-vibration table 1001 for mounting a precision equipment, such as an X-Y stage, is supported by an anti-vibration mount 1002. Generally, a plurality of anti-vibration mounts are used to support the anti-vibration table 1001. For instance, if the anti-vibration table 1001 has a rectangular shape, four anti-vibration mounts are provided, one at each corner of the anti-vibration table 1001. In FIG. 8, one of those anti-vibration mounts is shown as the anti-vibration mount 1002.
The main configuration elements of the anti-vibration mount 1002 are as follows. The anti-vibration mount 1002 supports the anti-vibration table 1001 upwardly using a mechanical spring or an air spring (not shown), thereby isolating the anti-vibration table 1001 from vibrations of the setting floor. A rising member 1032 is connected to the anti-vibration table 1001 by a joint bolt 1031 and rises with the anti-vibration table 1001, and a base member 1033 is placed on the setting floor. A vertical acceleration sensor 1034 for measuring acceleration of the anti-vibration table 1001 in the vertical direction and a horizontal acceleration sensor 1003 for measuring acceleration of the anti-vibration table 1001 in the horizontal direction are fixed on the rising member 1032. Further, a vertical actuator 1021 for applying driving force to the anti-vibration table 1001 in the vertical direction and a horizontal actuator 1022 for applying driving force to the anti-vibration table 1001 in the horizontal direction are provided between the base member 1033 and the rising member 1032. Besides the above elements, the anti-vibration mount 1002 in practice includes other mechanisms, such as a viscous member, although they are not shown. Since FIG. 8 is a conceptual view of the configuration of the active anti-vibration apparatus, only the main configuration elements necessary for actively controlling vibrations are shown. When supporting the anti-vibration table 1001 with a plurality of vibration mounts, each vibration mount has the same configuration as that of the anti-vibration mount 1002 in FIG. 8.
Next, a configuration of a control system for actively controlling vibrations of the anti-vibration table 1001 using the anti-vibration mount 1002 is explained. There are many suggested configurations of control systems to be adopted by an active anti-vibration apparatus. According to a typical configuration among those suggested configurations, acceleration of vibration of the anti-vibration table 1001 is measured and force proportional to a velocity, which is an integrated value of the acceleration, is applied to the anti-vibration table 1001, thereby applying dumping force to the anti-vibration table 1001. FIG. 8 shows a mechanism of such a control system. The sign of a measurement signal outputted from the vertical acceleration sensor 1034 is changed by a sign changer 1035, and the sign-changed signal is operated by an acceleration compensator 1036. The obtained compensation value is inputted to a power amplifier 1010. The power amplifier 1010 drives the vertical actuator 1021 in accordance with the input signal, thereby driving force in the vertical direction is applied to the anti-vibration table 1001. By performing negative feedback for vertical acceleration of the anti-vibration table 1001 as described above, damping force in the vertical direction is applied to the anti-vibration table 1001. Note, in a case where the vertical actuator 1021 is a voice coil motor, the acceleration compensator 1036 performs integration in order to generate a velocity signal from an acceleration signal, whereas in a case where the vertical actuator 1021 is an air spring, the acceleration compensator 1036 performs proportional action since the vertical actuator 1021 itself has integrating characteristics. The same operation as described above is performed in the horizontal direction. Namely, the sign of a measurement signal outputted from the horizontal acceleration sensor 1003 is changed by the sign changer 1035, and the sign-changed signal is operated by the acceleration compensator 1036. The obtained compensation value is inputted to the power amplifier 1010. The power amplifier 1010 drives the horizontal actuator 1022 in accordance with the input signal, thereby driving force in the horizontal direction is applied to the anti-vibration table 1001. By performing negative feedback for horizontal acceleration of the anti-vibration table 1001 as described above, damping force in the horizontal direction is applied to the anti-vibration table 1001. Note, in a case where the horizontal actuator 1022 is a voice coil motor, the acceleration compensator 1036 performs integration in order to generate a velocity signal from an acceleration signal, whereas in a case where the horizontal actuator 1022 is an air spring, the acceleration compensator 1036 performs proportional action since the actuator 1022 itself has integrating characteristics. In a case where the anti-vibration table 1001 is supported by a plurality of anti-vibration mounts, a configuration of a control system of each anti-vibration mount is the same as that of the anti-vibration mount 1002 in FIG. 8.
However, in the active anti-vibration apparatus controlled in the aforesaid method, parameters used in the control loop constituted for each anti-vibration member do not correspond one-to-one to movement of each of motion modes, such as parallel translation and rotation, of an anti-vibration table. Accordingly, it is not easy to design and adjust control parameters in good perspective in consideration of stability and performance of the control system. In the aforesaid system, it is very difficult to arbitrarily adjust motion characteristics of the anti-vibration apparatus by adjusting control parameters for improving vibration control performance, since control systems which are independently provided for a plurality of supporting portion of an anti-vibration table interfere with each other.
In order to overcome the aforesaid problems, anti-vibration apparatuses which perform vibration control for each of motion modes, such as translation and rotation, of an anti-vibration table and control methods therefor are suggested as disclosed in Control Apparatus for Anti-Vibration Table of Japanese Patent Application Laid-Open No. 6-181158, Control Apparatus for Vertical-Direction-Air-Spring Type Anti-Vibration Table of Japanese Patent Application Laid-Open No. 7-83276, for instance, and effectiveness of the apparatuses and the control methods has been confirmed. A control system, adopted by these apparatuses and methods, which performs motion mode independent control focuses on vibrations of rigid body motion of the anti-vibration table 1001 in general, whereas the control system as explained with reference to FIG. 8 focuses on vibrations of a portion of the anti-vibration table 1001 where the anti-vibration mount 1002 supports, in other words, local vibrations of the anti-vibration table 1001. The motion mode independent control is briefly explained below.
FIG. 9 shows a configuration of an anti-vibration apparatus which controls an anti-vibration table for each motion mode independently. In the anti-vibration apparatus, a motion mode extractor 1006 extracts information on respective motion modes, such as translation and rotation, of the anti-vibration table 1001 on the basis of outputs from vibration sensors 1003a, 1003b and 1003c, such as acceleration sensors, and a compensation calculation circuit 1007 calculates a compensation value for each motion mode. Then, a thrust distributor 1008 distributes the calculated compensation signals for the respective motion modes to actuators, provided on active anti-vibration mounts 1002a, 1002b, 1002c and 1002d, for applying control force to the anti-vibration table 1001. The actuators provided on the active anti-vibration mounts 1002a to 1002d are driven by driving circuits 1010a, 1010b, 1010c and 1010d, respectively.
In such the active anti-vibration apparatus, control parameters, which are conventionally determined by greatly depending upon trial-and-error method for each of the control loops of the anti-vibration table, can be designed and adjusted by focusing on motion modes of the anti-vibration table, namely, overall motion characteristics of the anti-vibration table. Thus, it is possible to adjust and design the control parameters used in the anti-vibration apparatus easily and rationally.
FIG. 10 shows a configuration of an anti-vibration apparatus adopting another motion mode independent control method. In FIG. 10, four corners of the anti-vibration table 1001, which is roughly expressed as a rectangular solid, are supported by the anti-vibration mounts 1002a to 1002d. The anti-vibration mounts 1002a to 1002d have an identical configuration, and in which of the four anti-vibration mounts 1002a to 1002d elements are included is distinguished by supplemental alphabets, a to d, attached behind reference numerals of configuration elements. The anti-vibration mount 1002a includes a vertical acceleration sensor 1034a, a horizontal acceleration sensor 1003a, a vertical actuator 1021a, and a horizontal actuator 1022a. An X-Y-Z orthogonal coordinate system is defined with respect to the anti-vibration table 1001 in such a manner that the origin of the X-Y-Z coordinate system is set at the center of gravity G of the anti-vibration table 1001, and X- and Y-axes are respectively parallel to two sides of the anti-vibration table 1001 having a rectangular solid, and a Z-axis is set in the normal direction to the anti-vibration table 1001. Accordingly, the displacement of the anti-vibration table 1001 can be expressed as motions of 6-degree-of-freedom constituted by translation x along the X-axis, translation y along the Y-axis, translation z along the Z-axis, rotational motion .theta.x about the X-axis, rotational motion .theta.y about the Y-axis, and rotational motion .theta.z about the Z-axis. In the motion mode independent control performed by the apparatus shown in FIG. 10, negative feedback for acceleration is performed by each of motion modes of 6-degree-of-freedom and damping force is applied to the anti-vibration table 1001.
The motion mode extractor 1006 calculates acceleration of each of the motion modes, namely, acceleration of the translation Ax along the X-axis, acceleration of the translation Ay along the Y-axis, acceleration of the translation Az along the Z-axis, angular acceleration of the rotation motion A.theta.x about the X-axis, angular acceleration of the rotation motion A.theta.y about the Y-axis, and angular acceleration of the rotation motion A.theta.z about the Z-axis, on the basis of measurement signals from the vertical acceleration sensors 1034a to 1034d and the horizontal acceleration sensors 1003a to 1003d, then outputs the calculated results. Then, the sign of acceleration of each of the motion modes is inverted by the sign changer 1035, the sign-inverted signal is operated by the acceleration compensator 1036, thereby generating the compensation value for each motion mode. The thrust distributor 1008 generates driving instructions for the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d on the basis of the inputted compensation values, and transmits them to the power amplifier 1010. The thrust distributor 1008 generates driving instructions for the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d so that driving force, generated by the acceleration compensator 1036 for the respective motion modes, to be applied to the anti-vibration table 1001 corresponds to the total force generated by the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d. The power amplifier 1010 drives the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d in accordance with the inputted driving instruction signals, thereby applying the driving force to the anti-vibration table 1001. As described above, in the motion mode independent control, negative feedback for acceleration is performed for each of a plurality of motion modes, thereby applying damping force to the anti-vibration table 1001 to restrain vibrations of the anti-vibration table 1001. Note, in a case where the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d are voice coil motors, the acceleration compensator 1036 performs integration in order to generate a velocity signal from an acceleration signal, whereas in a case where the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d are air springs, the acceleration compensator 1036 performs proportional action since the actuators themselves have integrating characteristics.
An arrangement of the anti-vibration mounts 1002a to 1002d will be explained. As for the vertical direction, measurement directions of the vertical acceleration sensors 1034a to 1034d and operation directions of the vertical actuators 1021a to 1021d match the Z-axis direction. Regarding horizontal directions, the anti-vibration mounts 1002a to 1002d are arranged in such a manner that the measurement directions of the horizontal acceleration sensors 1003a and 1003c and operation directions of the horizontal actuators 1022a and 1022c of the anti-vibration mounts 1002a and 1002c match the X-axis direction, and measurement directions of the horizontal acceleration sensors 1003b and 1003d and operation directions of the horizontal actuators 1022b and 1022d of the anti-vibration mounts 1002b and 1002d match the Y-axis direction. The reason for arranging the anti-vibration mounts 1002a to 1002d as described above is that all the motion modes of 6-degree-of-freedom of the anti-vibration table 1001 can be measured by the vertical acceleration sensors 1034a to 1034d and the horizontal acceleration sensors 1003a to 1003d, and the vibrations of the anti-vibration table 1001 can be controlled by the vertical actuators 1021a to 1021d and the horizontal actuators 1022a to 1022d.
In a method for controlling an anti-vibration table for each motion mode, it is necessary to detect movement of the anti-vibration table by each motion mode in satisfactory precision and properly distribute thrust to actuators in correspondence with driving instruction signals of respective motion modes. In a practical anti-vibration apparatus, however, due to spatial and mechanical limitation for connecting the anti-vibration mounts to the anti-vibration table or an equipment, for instance, to be isolated from vibrations, there are cases in which rigidity of connecting portions between the anti-vibration mount, and the vibration sensor and an actuator, and joint rigidity between the anti-vibration mount and the anti-vibration table are not secured satisfactorily. For this reason, various local vibrations may occur at various portion around the anti-vibration table.
In the method for controlling the anti-vibration table for each motion mode, vibrations are controlled by focusing on rigid body motion modes of the anti-vibration table, or on rigid body motion modes and several deformed motion modes having natural frequencies in a relatively low frequency region. However, when local vibrations of deformed motion modes having natural frequencies of relatively high frequencies occur around the vibration sensor or the actuator, or in the anti-vibration table itself, a vibration signal of each motion mode of the anti-vibration table may not be correctly extracted due to the effect of the local vibrations, further, thrust may not be correctly distributed in correspondence with a compensation signal for each motion mode. When vibration control is performed by mainly focusing on rigid body motion modes having natural frequencies in a lower frequency region under the aforesaid situation, local vibrations of very high frequencies exert a bad influence on control performance. If the worst comes to the worst, so-called "spill-over" unstability occurs and a very big vibration would be excited in the anti-vibration table.
The "spill-over" is a phenomenon in which the subject of vibration control becomes unstable due to an effect of a motion mode of a higher degree than the degrees of vibration modes which the vibration control expects. For example, in a system capable of controlling motion modes of n-degrees, an effect of motion mode of (n+1)-degrees cannot be reflected in a control logic, and unstabilizes the system as disturbance.
By using the proper number of vibration sensors and actuators so as to be able to extract signals of a large number of motion modes of various local vibrations and distribute thrust in correspondence with the compensation signals of the large number of motion modes, it is possible to avoid the aforesaid problem by performing appropriate compensation calculations and vibration control. However, as an industrial device, it is not realistic to use too many vibration sensors and actuators since cheaper manufacturing cost is required.
As described above, the active anti-vibration apparatus measures vibrations of the anti-vibration table, which is to be supported, by the acceleration sensors, and drives the actuators in accordance with the measurement signals, thereby controls vibrations of the anti-vibration table. Conventionally, an acceleration sensor is fixed on the rising member 1032 of the anti-vibration mount 1002, as shown in FIG. 8. The rising member 1032 is joined to the anti-vibration table 1001 by the joint bolt 1031, thus, vibrations of the anti-vibration table 1001 theoretically match vibrations of the rising member 1032. Therefore, it should be possible to measure the vibrations of the anti-vibration table 1001 by the acceleration sensor fixed to the rising member 1032 without any problem. However, vibrations of the anti-vibration table 1001 do not match vibrations of the rising member 1032 in practice. A measurement signal of the acceleration sensor which is fixed to the rising member 1032 includes not only vibrations of the anti-vibration table 1001 but also vibrations peculiar to the rising member 1032 which do not occur in the anti-vibration table 1001. Moreover, the peculiar vibrations of the rising member 1032 cannot be ignorable when its acceleration is considered with respect to that of vibrations of the anti-vibration table 1001.
The peculiar vibration of the rising member 1032 is due to low mechanical rigidity of the rising member 1032. In a semiconductor exposure apparatus, the mass and rigidity of the anti-vibration table 1001 for mounting precision equipment, such as an X-Y stage for exposure operation and an illumination mechanism, is sufficiently large. In contrast, the rising member 1032 is thinner and has much lower rigidity compared to the anti-vibration table 1001. Further, the rigidity of the joint portion between the rising member 1032 and the anti-vibration table 1001 by the joint bolt 1031 may also cause a problem. Since the rising member 1032 is joined to the anti-vibration table 1001 in a state as a cantilever, as shown in FIG. 8, when the joint rigidity is low, vibrations having a node at the joint portion may occur in the rising member 1032. To increase the thickness of the rising member 1032 for securing enough rigidity is not realistic since that results in increase in the both size and weight of the anti-vibration mount 1002.
Furthermore, joint rigidity of the joint portion cannot be increased infinitely, and is limited to a certain level. Therefore, it is practically impossible to avoid the problem in which the peculiar vibrations of the rising member 1032 occur and the vibrations of the rising member 1032 do not match vibrations of the anti-vibration table.
When the peculiar vibrations of the rising member 1032 is included in measurement signals by the acceleration sensors, performance of the active anti-vibration apparatus is greatly debased. As shown in FIG. 8, a typical control system of the active anti-vibration apparatus is to give damping force to the anti-vibration table 1001 by performing negative feedback to acceleration. If the vibrations of the rising member 1032 to which an acceleration sensor is fixed do not match the vibrations of the anti-vibration table 1001 which is to be controlled, it is impossible to properly apply damping force to the anti-vibration table 1001. Furthermore, since the mass of the rising member 1032 is generally much smaller than that of the anti-vibration table 1001, the peculiar vibrations of the rising member 1032 at resonance is much larger than the vibrations of the anti-vibration table 1001 when accelerations of vibrations are considered. For this reason, it is difficult, in most cases, to completely remove components originated from the peculiar vibrations of the rising member 1032 from measurement signals from the acceleration sensors even when the measurement signals are processed by a band-pass filter constituted by a low-pass filter and a high-pass filter. If vibration of acceleration measured by the acceleration sensor is affected by resonance of the rising member 1032, sufficient vibration control cannot be achieved, thereby it is not possible for the active anti-vibration apparatus to fully utilize its vibration control ability. Furthermore, when the actuator is driven in resonant range of the rising member 1032 in order to cancel the resonant vibration of the rising member 1032, there is a danger that resonant vibration of the rising member 1032 is further stimulated, which eventually causes so-called spill-over phenomenon that unstabilizes the control system.
Further, in the active anti-vibration apparatus realizing the motion mode independent control as shown in FIG. 10, there is a problem in which motion mode independent control cannot be correctly realized because of peculiar vibrations occurring on the rising member 1032. In the motion mode independent control, accelerations of motion modes of 6-degree-of-freedom, Ax, Ay, Az, A.theta.x, A.theta.y, and A.theta.z, of the anti-vibration table 1001 are generated by performing calculation on the basis of the measurement signals from respective acceleration sensors. Therefore, if a vibration component other than the vibrations of the anti-vibration table is included in the measurement signals, the correct calculation for extracting the motion modes cannot be performed. In other words, a control system which performs negative feedback for acceleration of each motion mode cannot be configured. Under this situation, it is not possible for the active anti-vibration apparatus to operate at full performance for controlling and removing vibrations. This debasement of performance of the active anti-vibration apparatus directly causes debasement of performance of an overall apparatus, such as a semiconductor exposure apparatus.